This invention relates to optical apparatus for measuring the thickness or chemical composition of partially transparent films, which are either self-supporting or attached to a reflecting substrate. Such equipment is useful in laboratory settings, and is also highly valuable for process control functions. It is particularly desired to provide an industrial coating monitor which will function on-line during the manufacturing process to measure continuously the thickness of plastic coatings on metal surfaces.
Infra-red absorption techniques have long been used to identify and analyze a wide variety of materials. In particular, techniques have been developed for determining the thickness and composition of film by measuring and comparing the transmission of radiation at two or more wavelengths. (Miller and Mounsey Paper regarding "On-Line Measurement Of Blown & Cast Film-Profile and Average Gauge Utilizing Infrared Techniques" in 1971 TAPPI Plastics-Paper Conference). Most commonly, these techniques involve a source of infrared radiation on one side of the film and a detector on the other.
There are many measurement situations in which it is either not convenient or not possible to place the source and receiver on opposite sides of the film. One example would be an organic polymer coating on an opaque substrate such as paper or metal. In such cases, it is necessary to place the detector on the same side of the target as the source and analyze the radiation which is transmitted through the coating and reflected back by the substrate. In the past, the operation of such reflective film gauges has been limited by the fact that the front surface of the coating reflects several percent of the radiation which strikes it. The radiation reflected by this first surface interferes with the radiation transmitted through the film and reflected by the substrate, in such a way as to give rise to a variation in the measured signal. This is dependent on the optical phase shift in the film and hence on both the film thickness and the wavelength of the radiation.
Designs have previously been proposed to minimize the problem of first surface reflection. For diffuse substrates such as paper, a fairly simple design has been found to be adequate (Brunton U.S. Pat. No. 3,693,025). In this case, the substrate surface is rough, whereas that of the coating is relatively smooth. As a result, the substrate reflects diffusely, i.e., in all directions; whereas the surface reflection is specular, i.e., confined to a narrow range of directions. The first surface reflection can be avoided by placing the radiation source and the detector subsystem at positions so that the optical rays between them and the surface make different angles with the normal to the surface. The specular reflection from the first surface will then miss the detector, but enough of the diffuse second surface reflection will be received to allow system operation.
Techniques have also previously been proposed for reducing the first surface effects in the case of coatings on specular substrates (Brunton U.S. Pat. No. 3,631,526). In this case, both the first and second surface reflections are specular and thus cannot be distinguished on the basis of their directionality. The three approaches proposed amount to: (1) averaging over a range of incidence angles; (2) using adjacent wavelengths; and (3) averaging over a range of wavelengths. All three of these approaches have been found experimentally to yield too little improvement to be useful for many applications of interest. The following items include all other prior art known to applicant: Brun Infra-Gauge BF-100 Brochure; Brunton U.S. Pat. Nos. 3,597,616; Brunton et al 3,790,796; and Van Horne et al 3,803,414.